Joint implants have been manufactured from polyolefins, particularly high density and ultrahigh molecular weight polyethylene (UHMWPE). Today, almost all polymeric joint implants are manufactured from UHMWPEs with molecular weight in the range of 2 to 6 million grams/mol. The viscosity of these polymers generally increases with increasing molecular weight and their melt flow rate (MFR) generally decrease. At these molecular weights, UHMWPE resin can only be consolidated into solid forms by methods of compression molding (for example, compression molding, direct compression molding, hot isostatic molding) and specialized extrusion (ram extrusion). These consolidated forms can be used as end-products such as joint implants and end-products can be machined further from these consolidated solid forms such as sheets, bars or rods. Alternatively, preforms, i.e. transient solid forms before the end-product, can be machined from these consolidated forms and further treated before a final end product can be made by for example, machining.
The mechanical properties of the consolidated solid forms of polymeric resin are dependent on the consolidation method and conditions. In addition, one or more of the mechanical properties can be enhanced further by further treatment of the consolidated form, for example by peroxide cross-linking (U.S. Application No. 61/756,596), high pressure crystallization (U.S. Pat. Nos. 8,420,000; 8,425,815; 8,426,486) or high temperature melting (U.S. Patent Publication No. 2012/0041094). These methods may include exposure to temperatures above the melting point of polyethylene, especially temperatures close to or greater than 200° C. for prolonged periods of time.
During the consolidation of UHMWPE, there may be variable amounts of dissolved and trapped gases in the polymer matrix. The amount of trapped gas may depend on the relative rates of cooling and crystallization of different parts of the pressed form of the polymer in the last steps of consolidation. Finally, high temperature exposure of such a consolidated form results in the expansion of the trapped gases and can cause defects in the samples (FIGS. 1 and 2). These defects can be exacerbated by molecular processes accompanying high temperature exposure.
This invention discloses methods of manufacturing peroxide cross-linked and high temperature melted polymeric material total joint implants, where peroxide cross-linking is limited to a finite thickness on the surfaces where wear resistance is desired and the high temperature melted polymeric material makes up the rest of the joint implant. This is essentially a medical implant with non-uniform properties. The surface has good wear resistance and the bulk has good mechanical properties.
This invention also discloses a method of manufacturing of high temperature melted consolidated UHMWPE for total joint implants in more shapes and sizes (FIG. 3). The invention comprises a method of making an implant, wherein the UHMWPE powder is made into a ‘pre-molded green’ shape by sintering with and without pressure and with or without elevated temperature without completely consolidating the material, exposing this pre-molded form to high temperature, then completely molding the material into a consolidated form from which implants can be machined, by irradiating the consolidated form, machining an implant, packaging and sterilizing the implant.